Flow Trays

Double pass A split-flow tray with two liquid flowpaths on each tray. Each path handles half of the total liquid flow. 

Select single cross-flow tray, segmental downcomers, and straight weirs, with weir length equaling 77% of tower diameter. The do vnflow segment is 12.4% of the tower area. 

Three principal types of cross-flow tray are used, classified according to the method used to contact the vapour and liquid. 

Figure 11.21. Liquid flow patterns on cross-flow trays, (a) Single pass (b) Reverse flow (c) Double pass...

Fig. 9.4 Sieve trays with downcomers for liquid systems with (a) high interfacial tension and dual-flow trays for liquid systems and (b) with low interfacial tension. In the case of downcomers, only the phase to be dispersed flows through the holes. The droplets are formed by jet disintegration. In dual-flow trays, both hquids flow through the same holes alternately. The larger drops spUt because of colhsion with the tray.

The main kinds of cross flow trays with downcomers in use are sieve, valve, and bubblecap. 

Figure 17.11. Types of contactors for reacting gases with liquids many of these also are suitable for reacting immiscible liquids. Tanks (a) with a gas entraining impeller (b) with baffled impellers (c) with a draft tube (d) with gas input through a rotating hollow shaft, (e) Venturi mixer for rapid reactions, (f) Self-priming turbine pump as a mixer-reactor, (g) Multispray chamber. Towers (h) parallel flow falling film (i) spray tower with gas as continuous phase (j) parallel flow packed tower (k) counter flow tray tower. (1) A doublepipe heat exchanger used as a tubular reactor.

Dual-Flow Trays These are sieve trays with no downcomers (Fig. 14-27b). Liquid continuously weeps through the holes, hence their low efficiency. At peak loads they are typically 5 to 10 percent less efficient than sieve or valve trays, but as the gas rate is reduced, the efficiency gap rapidly widens, giving poor turndown. The absence of downcomers gives dual-flow trays more area, and therefore greater capacity, less entrainment, and less pressure drop, than conventional trays. Their pressure drop is further reduced by their large fractional hole area (typically 18 to 30 percent of the tower area). However, this low pressure drop also renders dual-flow trays prone to gas and liquid maldistribution. 

In general, gas and liquid flows pulsate, with a particular perforation passing both gas and liquid intermittently, but seldom simultaneously. In large-diameter (>2.5-m, or 8-ft) dual-flow trays, the pulsations sometimes develop into sloshing, instability, and vibrations. The Ripple Tray is a proprietary variation in which the tray floor is corrugated to minimize this instability. 

With large holes (16 to 25 mm), these trays are some of the most fouling-resistant and corrosion-resistant devices in the industry. This defines their main application highly fouling services, slurries, and corrosive services. Dual-flow trays are also the least expensive and easiest to install and maintain. 

A wealth of information for the design and rating of dual-flow trays, much of it originating from FRI data, was published by Garcia and Fair [Ind. Eng. Chem. Res. 41 1632 (2002)]. 

Low liquid rates. With the aid of serrated weirs, splash baffles, reverse-flow trays, and bubble-cap trays, low liquid rates can be handled better in trays. Random packings suffer from liquid dewetting and maldistribution sensitivity at low liquid rates. 

Figure 6.1 Continued) Common tray types, (c) Sieve tray id) dual flow tray. [Pari e courtesy ofGlitsch, Inc,] [Peal d courtesy of Fractionation Research Inc. (FRI)-l...

Type Sieve tray Valve tray Bubble-cap tray Dual-flow tray... 

The froth height in the center downcomers in the bottom section is only slightly above 30 percent, and increasing the downcomer clearance will suffice to overcome the problem. However, this is unlikely to suffice for the side downcomers in the bottom section. In this example, idle clearance under the downcomer will be increased to 2.0 in in the center downcomers, and to 2.25 in in the side downcomers. The weir height on idle cenler-to-side flow trays in the bottom section will be lowered to 1.5 in to lower tray pressure drop. 

The FRI data for dualflow trays have been used to form a tentative design method (Garcia and Fair, 2002). Data for Turbogrid trays have been included in the model. A generalized chart for predicting allowable vapor velocity, similar to that for cross flow trays given in Fig. 13.32(b), is included. 

Cross-flow trays are also classified according to the number of liquid passes on the plate. The design shown in Figure 11.23a is a single-pass plate. For low liquid flow... 

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